Perhaps you may be interested in Librem 5 or Pinephone, both of which have hardware kill switches for modem and available schematics. The latter even has most of the modem software freed.

Those devices have atrocious security at a hardware, firmware and software level. Their microphone kill switch also doesn't prevent audio recording. They aren't open hardware despite many attempts to mislead people with the marketing.

> The latter even has most of the modem software freed.

Pinephones have entirely closed source baseband firmware. They use a highly unusual cellular radio which includes both an incredibly outdated Qualcomm baseband processor with atrocious updates and security combined with an extremely outdated proprietary fork of Android running on an extra CPU core which isn't present in any mainstream smartphone. It's only replacing the unusual extra OS which has been done. That whole component doesn't exist on other smartphones and the only reason it's possible to replace it is because the whole radio has absolutely atrocious security. The radio is connected via a far higher attack surface USB connection providing far less isolation for the OS and the USB connection can be used to flash the proprietary Android OS via the fastboot protocol. The baseband firmware itself doesn't have any replacement available.

Security theater, it has absolutely no use. If you can't trust your hardware that it won't actively listen to the microphone without your knowledge and permission then what are you even doing with that device?!